With the threat of energy shortage and global warming caused by environmental pollution, recent times have seen an increasing interest in new clean energy sources. In particular, there is a growing interest in and active research focused on hydrogen energy sources. Preferably, such hydrogen sources should be produced by solar energy, wind energy, biomass, and other renewable energy sources.
Due to technological and economical limitations, however, the steam methane reforming (SMR) method is currently the most widely used, and in recent times, a method of mass-producing hydrogen has been proposed that uses coal, an existing type of fossil fuel. The advantages of using coal are that there are abundant coal deposits, the cost is low, and there are coal deposits distributed all over the world.
Referring to FIG. 1, hydrogen can be produced by way of a water-gas shift (WGS) reaction that reacts the CO of a synthesis gas obtained by the gasification of coal with steam (CO+Steam←→H2+CO2). However, the gases produced by the WGS reaction may include many impurities besides the desired final product of hydrogen. Thus, in order to obtain hydrogen of a high purity level, the impurities may have to be removed by a separation refining process such as condensation and pressure swing adsorption (PSA), but during this process, 10-25% of the produced hydrogen may be consumed, and waste gases may be produced including a large quantity of carbon dioxide.
In order to resolve the problems and limitations of the existing WGS reaction, the sorption-enhanced WGS (SE-WGS) reaction is recently being developed. With the SE-WGS reaction, the reaction by-product of carbon dioxide can be removed by sorption in a column at the same time a WGS reaction is being performed, as illustrated in FIG. 2, to simplify the overall process. Also, Le Chatelier's principle is used to overcome thermodynamic limitations, and the reaction is shifted towards the product side, thereby making it possible to directly produce high-purity hydrogen of a level applicable to fuel cells (CO<10 ppm) without a separate refining process.
Currently, there are efforts for commercializing the SE-WGS reaction in various countries, but prior research has mostly focused on high-temperature CO2 adsorbents for application to the SE-WGS reaction and on enhancing the performance of the high-temperature CO2 adsorbents, and there is generally less research being performed on the process itself.
In comparison, the sorption-enhanced steam methane reforming (SE-SMR) reaction entails both a steam reforming reaction using methane and a sorption-based CO2 removal reaction that are performed simultaneously, making it possible to increase the reaction rate for hydrogen production as well as the thermodynamic yield.
Also, the SE-SMR reaction is a complex reaction in which a CO2 sorption reaction, which is an exothermic reaction, and a steam reforming reaction, which is an endothermic reaction, occur simultaneously to allow the possibility of reduced energy losses. The SE-SMR reaction can be performed at a temperature of 450 to 550° C., which is lower than the temperature range for an existing steam reforming reaction. The mechanisms in a SE-SMR reaction are as follows.CH4(g)+2H2O(g)CO2(g)+4H2(g)H298=164.8 kJ/mol
CO2 sorptionCO2(g)+CaO(s)CaCO3(s)H298=−178.2 kJ/mol
Sorption enhanced Steam Methane Reforming (SESMR)CH4(g)+2H2O(g)+CaCO(s)4H2(g)+CaCO3(s)H298=−13.4 kJ/mol
To increase the amount of production of high-purity hydrogen using such SE-SMR process, there is a need for an adsorbent having a fast sorption rate and high sorption capacity, so that any CO2 may be adsorbed and removed as soon immediately after it is produced.